Bone marrow edema (which may also be called bone bruising) is the presence of excess fluids in the bone marrow. Bone marrow edema pattern (BMEP) is a visible feature in a medical image that may indicate the presence of bone marrow edema. In some circumstances, BMEP may be associated with lytic bone lesions. Lytic bone lesions may be caused by certain bone tumors, amongst other possible causes. Visualizing BMEP can help diagnose lytic bone lesions, since relatively few bone tumors have been found to be associated with BMEP.
It is known to identify BMEP in magnetic resonance (MR) imaging. Studies have also investigated the use of computed tomography (CT) bone subtraction to identify BMEP, and compared results obtained using CT bone subtraction to results obtained using MR.
CT bone subtraction is a form of Digital Subtraction Angiography. In Digital Subtraction Angiography, a contrast agent is introduced to the vascular system of a patient. The contrast agent increases the intensity of blood as viewed in a CT image. Image data is acquired with and without the contrast agent present, and non-enhanced image data is subtracted from contrast-enhanced image data to remove features that are common to the two images (for example, bone) and to emphasize regions of contrast. In perfusion studies, multiple images may be taken at predetermined intervals to characterize the flow of contrast-enhanced blood through the vascular system.
If images that are required to be subtracted are not or may not be aligned (for example, if the patient may have moved between the first image and the second image) then registration may be used to align the sets of image data before subtraction. Registration can be used to determine a relationship between the coordinate systems of the first image data set and the second image data set, such that each anatomical location in the first image data set is mapped to the same anatomical location in the second image data set. Registration may be performed manually or automatically using known analysis techniques. Different types of registration may be used, for example rigid, affine, or non-rigid.
A rigid registration in this context may be considered to be a registration in which the coordinates of data points in one data set are subject to rotation and translation in order to register the data set to another data set. An affine registration in this context may be considered to be a registration in which the coordinates of data points in one dataset are subject to rotation, translation, scaling and/or shearing in order to register the dataset to another dataset. Thus, a rigid registration may be considered to be a particular type of affine registration.
Non-rigid registrations produce non-linear transformations, in which the coordinates of data points in one dataset are subject to flexible deformations in order to register the data set to another data set. Non-linear transformations may be defined using dense vector fields, defining an individual displacement for each voxel in a three-dimensional data set. Non-linear transformations may also be defined using other fields or functions, for example using B spline functions or thin plate spline functions.
In studies, CT images have been obtained for a set of patients, each patient having a lytic bone lesion, with the CT images being obtained from CT perfusion studies performed in a large detector CT scanner. A faint flushing of tissue may be seen while the contrast agent is introduced. In some circumstances, the effect of the contrast agent on intensity has been found to be of the order of 30 HU (Hounsfield units) and so may be difficult to distinguish.
Based on registration and subtraction of pre- and post-contrast CT image data sets using either non-rigid or rigid registrations, it has been found in some cases, in the context of CT perfusion studies on patients suffering from lytic bone lesions, that non-rigid registration gives results that may initially appear in some ways to be better than those of rigid registration, with better image quality and a reduction in subtraction artifacts when compared with rigid registration. However, non-rigid registration may result in the incorrect shrinkage or expansion of an area of BMEP (for example, associated with a tumor) after registration. Enhancement may also be reduced when compared with rigid registration. When assessing the contrast between the BMEP regions and region of bone marrow that appear normal, rigid registration may result in a higher contrast than is achieved using non-rigid registration.